The invention relates to a vacuum belt conveyor equipped with lateral guidance for a web forming machine, the vacuum belt conveyor being intended for the transportation of a web threading tail and comprising                a frame construction and at least two pulleys or rolls therein,        an air-permeable belt loop arranged around the rolls, and        guiding equipment for providing lateral guidance and thus for keeping theweb threading tail on the vacuum belt conveyor, in which there is arranged a vacuum effect for the section of the belt loop transporting the web threading tail.        
WO publication No. 03018909 sets forth a vacuum belt conveyor, in which, as in other known vacuum belt conveyors, the travel of the web threading tail on the surface of the belt loop is based on the friction force between the web threading tail and the belt loop. In addition, the friction force is proportional to the intensity of vacuum arranged inside the belt loop. In practice, increasing the vacuum increases the friction force, the direction of which is the same as that of the belt loop. In addition, the web threading tail and the belt loop usually have equal travel directions, in which case the web threading tail is not subjected to any cross-directional force. Consequently, the web threading tail can move in the cross direction relatively easily before being subjected to the returning cross-directional force caused by the deviation in the travel directions of the web threading tail and the belt loop.
Particularly in long belt conveyors, side walls are additionally used as guiding devices for keeping the web threading tail on top of the belt loop. In other words, the side walls are used to help prevent the cross-directional movement of the web threading tail. In practice, however, it has been noticed that the preventive and guiding effect of a side wall is insufficient for preventing the cross-directional movement irrespective of a high side wall. Furthermore, between the web threading tail and the side wall there remains an air layer such that the web threading tail curls up and typically rises upwards along the side wall. Consequently, the web threading tail is at least partly out of the range of the vacuum effect and also otherwise in an incorrect position with respect to the frame structure. This may lead to failed tail threading or at least to malfunctions. Side walls also collect loose material and affect disadvantageously the travel of the web threading tail also in other ways without performing however in a planned way. On the other hand, belt conveyors are used without side walls as well, in which case the previously mentioned problems are avoided, but at the same time even the slight guiding effect of the side walls is lost.